This invention relates to centrifugal blowers, such as those used for heating, ventilating, and air conditioning (HVAC).
A basic design feature of a centrifugal impeller is the angle that the blade trailing edge makes with a tangent to the impeller. This angle is called the blade trailing edge angle. Backward curved impellers have blade trailing edge angles less than 90 degrees, while forward curved impellers have blade trailing edge angles in excess of 90 degrees. Another basic design feature is the blade camber. Blade camber is defined as the ratio of the perpendicular distance from the meanline to the blade chord, to the length of the blade chord itself.
Two important performance characteristics of a centrifugal impeller are its non-dimensional flow and pressure capability; i.e., the performance capability of the impeller normalized on diameter and operating speed. Backward curved impellers typically run faster or are larger in diameter than a forward curved impeller running at the same operating point, and backward curved impellers typically operate at higher static efficiencies. Forward curved impellers operate at lower efficiencies, but can either run more slowly or be smaller in diameter at the same operating point.
In automotive climate control applications for centrifugal blowers, the impeller may be located within the cabin adjacent to the occupants, so that noise and vibration control are important. In these and various other applications, centrifugal blowers should operate not only with low noise and vibration, but they also should operate with high efficiency over a span of operating conditions in a relatively small volume package. For example, in automotive HVAC systems, several functions may be achieved by opening and closing duct passages, and flow resistance typically is greatest in heater and defrost conditions and least in air conditioning mode. Impeller output should be strong in all operating conditions, if at all possible, and impeller operation should be quiet at all operating points. With respect to backward curved impellers in particular, high resistance heater and defrost modes may cause particular noise problems, which may be termed a low frequency roar.
Yapp, U.S. Pat. No. 4,900,228 discloses rearwardly curved centrifugal impeller blades with xe2x80x9cSxe2x80x9d shaped camber. One embodiment discloses a maximum camber which is 5% of blade chord, and a blade exit angle between 50 and 60 degrees from the impeller tangent.
This invention combines characteristics of both backward curved and forward curved impellers to gain the advantages of both. The leading edge geometry is similar to that of a conventional backward curved impeller, but the camber and trailing edge angles are much higher.
In general, one aspect of the invention features a centrifugal impeller whose radially extending blades are characterized by:
a) a high positive camber at a radially inward region of the blade, for example, a maximum camber value of at least 7% and even 10% or more of the blade chord, and the maximum camber occurs at x/C less than 0.5, and preferably at x/C less than 0.4;
b) a large trailing edge angle, for example, a trailing edge that forms an angle of at least 70 degrees with the impeller tangent; and
c) a top shroud surface, which is shapedxe2x80x94i.e., it has curvature in a plane that contains the impeller axis (the xe2x80x9cradial directionxe2x80x9d, FIG. 3)xe2x80x94to help control flow diffusion and help eliminate stall, and which is connected to the impeller blades and covers at least a substantial portion of the chord length of the impeller blades. The shroud can also sometimes incorporate an inlet lip to help the flow enter the impeller blades with relatively low turbulence, helping reduce the possibility of stall.
In preferred embodiments, the chord is long, typically at least 15% or even 20% of the impeller diameter. Also in preferred embodiments, the impeller has a cylindrical area ratio between 1.0 and 1.5 , the blade leading edge radius is at least 0.8% of the blade chord length, and at least one impeller component is injection molded plastic. The impeller diameter is between 75 and 300 millimeters, and the ratio of blade number to impeller diameter in millimeters is at least 0.15 and is more preferably at least 0.2.
The invention controls not only low frequency roar, but also overall noise and vibration under given operating conditions. The blade leading edges are aligned with the incoming airflow to limit the aerodynamic loading there, preventing immediate flow separation. The blades are highly cambered and have a relatively high blade trailing edge angle, enabling the impeller to have high non-dimensional flow and pressure capability. The blade trailing edge angle approaches that of a conventional forward curved impeller, but the design of the hub, the curved shroud surfaces and greater blade chord length allow diffusion (the conversion of kinetic energy into static pressure) to occur. A high blade number also helps to control the diffusion process. The invention is particularly suitable for automotive applications because it can provide performance similar to conventional backward curved impellers, but at a lower operating speed or smaller diameter.
Other features and advantages of the invention will be apparent to those skilled in the art from the following description of the preferred embodiment and from the claims.